The separation of one or more gases from a complex multicomponent mixture of gases is necessary in a large number of industries. Such separations currently are undertaken commercially by processes such as cryogenics, pressure swing adsorption and membrane separations. In certain types of gas separations, membrane separations have been found to be economically more viable than other processes.
In a pressure driven gas membrane separation process, one side of the gas separation membrane is contacted with a complex multicomponent gas mixture and certain of the gases of the mixture permeate through the membrane faster than the other gases. Gas separation membranes thereby allow some gases to permeate through them while serving as a barrier to other gases in a relative sense. The relative gas permeation rate through the membrane is a property of the membrane material composition and its morphology. It has been suggested in the prior art that the intrinsic permeability of a polymer membrane is a combination of gas diffusion through the membrane, controlled in part by the packing and molecular free volume of the material, and gas solubility within the material. Selectivity is determined by dividing the permeabilities of two gases being separated by a material. It is also highly desirable to form defect-free dense separating layers in order to retain high gas selectivity.
In gas separations, it is also advantageous to use membranes which possess the desired properties of selectivity, flux, and mechanical strength to withstand prolonged operation at high temperatures and pressures. Furthermore, in order for gas separations to be commercially viable, it is advantageous to use membranes that can be manufactured in large quantities at high product quality, and which can be inexpensively assembled into a permeator.
The preparation of commercially viable gas separation membranes has been greatly simplified with asymmetric membranes. Asymmetric membranes are prepared by the precipitation of polymer solutions in solvent-miscible nonsolvents. Such membranes are typified by a dense separating layer supported on an anisotropic substrate of a graded porosity and are generally prepared in one step. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; and 4,512,893. U.S. Pat. No. 4,717,394 shows preparation of asymmetric separation membranes from selected polyimides.
Composite gas separation membranes typically have a dense separating layer on a preformed microporous substrate. The separating layer and the substrate are usually different in composition. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,664,669; 4,689,267; 4,741,829; 2,947,687; 2,953,502; 3,616,607; 4,714,481; 4,602,922; 2,970,106; 2,960,462; and 4,713,292.
Composite gas separation membranes have evolved to a structure of an ultrathin, dense separating layer supported on an anisotropic, microporous substrate. These composite membrane structures can be prepared by laminating a preformed ultrathin dense separating layer on top of a preformed anisotropic support membrane. Examples of such membranes and their methods of manufacture are disclosed in U.S. Pat. Nos. 4,689,267; 4,741,829; 2,947,687; 2,953,502; 2,970,106; 4,086,310; 4,132,824; 4,192,824; 4,155,793; and 4,156,597.
Composite gas separation membranes are generally prepared by multistep fabrication processes. Typically, the preparation of composite gas separation membranes require first forming an anisotropic, porous substrate. This is followed by contacting the substrate with a membrane-forming solution. Examples of such methods are shown in U.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994. U.S. Pat. No. 4,756,932 shows forming composite hollow-fiber membranes by dip coating. Alternatively, composite hollow-fiber membranes may also be prepared by co-extrusion of multiple polymer solution layers, followed by precipitation in a solvent-miscible nonsolvent.
The hollow-fiber spinning process depends on many variables which may affect the morphology and properties of the hollow-fiber membrane. These variables include the composition of the polymer solution employed to form the fiber, the composition of fluid injected into the bore of the hollow-fiber extrudate during spinning, the temperature of the spinneret, the coagulation medium employed to treat the hollow-fiber extrudate, the temperature of the coagulation medium, the rapidity of coagulation of the polymer, the rate of extrusion of the fiber, takeup speed of the fiber onto the takeup roll, and the like.
A particular problem has been observed during the use of asymmetric and composite membranes for the separation of gas mixtures. In particular, the selectivity of gas separation membranes is significantly poorer for mixed gas separations than the corresponding ratio of the single gas permeabilities. For example, in a polyimide gas separation membrane with a feed stream containing 90% N.sub.2 and 10% CO.sub.2, at room temperature the selectivity for CO.sub.2 /N.sub.2 may be about 20; whereas the ratio of single gas permeability for CO.sub.2 to the permeability for N.sub.2 may be about 40. A need therefore exists for a membrane and a process of manufacture which avoids the above shortcomings of the prior art membranes and processes. The present invention is directed to improved membranes, particularly hollow-fiber membranes and their methods of manufacture. The invention, although applicable to membranes generally, has particular utility to hollow-fiber asymmetric and composite membranes. The improved hollow-fiber membranes are produced by varying the spinning solution formulations and the spinning process conditions to achieve the desired fiber morphology to provide fibers that have improved permeation properties and mechanical strength. The fiber membranes are especially useful in gas separations that require the use of high feed pressures.